1. Field of the Invention
The present invention relates to a high pressure discharge lamp such as a sodium-vapor lamp, a metal halide lamp or the like.
2. Background Art
Conventionally, a high pressure discharge lamp includes a vessel made of a non-conductive material which forms an inner space filled with an ionizable light-emitting material and a starting gas, the vessel having first and second opening portions at both ends thereof, and a non-conductive member inserted into the first opening portion of the vessel and having an outer diameter which is smaller than inner diameter of the first opening portion so as to form a gap between the vessel and the non-conductive member, the non-conductive member further having a hole. An electrode unit is inserted into the hole of the non-conductive member. The electrode unit has a first end which is exposed to the inner space of the vessel, and a second end which is exposed to outside of the vessel.
In this connection, GB-A-1421406, JP-U-52-19182 and U.S. Pat. No. 5,541,480 each discloses such a high pressure discharge lamp which is further provided with at least one starting electrode which serves to reduce the starting voltage.
In the high pressure discharge lamp according to GB-A1421406, the non-conductive member is in the form of a ring which is inserted into a gap between an envelope and a cap, and a metal layer is provided on an inner face of the ring and acts as a starting electrode. In this instance, since the ring is arranged between the envelope and the cap, it is necessary to seal the gap between the envelope and the ring, and also seal the gap between the ring and the cap. Such a structure is not very appropriate from the viewpoint of the corrosion resistance and the tightness property.
In the high pressure discharge lamp according to JP-U-52-19182 and U.S. Pat. No. 5,541,480, a linear conductive material such as carbon, platinum, palladium or the like is provided on a light-emitting portion thereof. This structure is not very appropriate because such a conductive material tends to decrease the luminous flux of the lamp.